Run the following java program, it will hang:

import java.io.IOException;
import java.net.Socket;
import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;
import java.security.KeyManagementException;
import java.security.NoSuchAlgorithmException;
import java.security.cert.CertificateException;
import java.security.cert.X509Certificate;

import javax.net.ssl.SSLContext;
import javax.net.ssl.SSLEngine;
import javax.net.ssl.TrustManager;
import javax.net.ssl.X509ExtendedTrustManager;

public class TestHttpStress {
    private HttpClient client;

    private URI target_get = new URI("https", null, <server name>, 8443,
            "/get", "size=" + 100000, null);

    public TestHttpStress() throws Exception {
        client = HttpClient.newBuilder().version(HttpClient.Version.HTTP_2)
                .sslContext(getSslContext()).build();
    }

    public static void main(String[] args) throws Exception {
        final int thread_num = Integer.parseInt(args[0]);
        TestHttpStress th = new TestHttpStress();
        for (int i = 0; i < thread_num; i++) {
            Thread t = new Thread(() -> th.run());
            t.start();
        }

    }

    public void run() {
        long i = 0;
        while (true) {
            int r = 0;
            try {
                r = get();
            } catch (Exception e) {
                e.printStackTrace();
            }
            System.out.println(r + " " + i++);

        }
    }

    private int get() throws IOException, InterruptedException {
        HttpRequest request = HttpRequest.newBuilder(target_get).GET().build();
        HttpResponse<byte[]> response = client.send(request,
                HttpResponse.BodyHandlers.ofByteArray());
        return response.body().length;
    }

    private SSLContext getSslContext()
            throws NoSuchAlgorithmException, KeyManagementException {
        TrustManager[] trustAllCerts = new TrustManager[] {
                new X509ExtendedTrustManager() {

                    @Override
                    public void checkClientTrusted(X509Certificate[] arg0, String arg1)
                            throws CertificateException {
                        // TODO Auto-generated method stub

                    }

                    @Override
                    public void checkServerTrusted(X509Certificate[] arg0, String arg1)
                            throws CertificateException {
                        // TODO Auto-generated method stub

                    }

                    @Override
                    public X509Certificate[] getAcceptedIssuers() {
                        // TODO Auto-generated method stub
                        return null;
                    }

                    @Override
                    public void checkClientTrusted(X509Certificate[] arg0, String arg1,
                            Socket arg2) throws CertificateException {
                        // TODO Auto-generated method stub

                    }

                    @Override
                    public void checkClientTrusted(X509Certificate[] arg0, String arg1,
                            SSLEngine arg2) throws CertificateException {
                        // TODO Auto-generated method stub

                    }

                    @Override
                    public void checkServerTrusted(X509Certificate[] arg0, String arg1,
                            Socket arg2) throws CertificateException {
                        // TODO Auto-generated method stub

                    }

                    @Override
                    public void checkServerTrusted(X509Certificate[] arg0, String arg1,
                            SSLEngine arg2) throws CertificateException {
                        // TODO Auto-generated method stub

                    }

                } };

        SSLContext sslContext = SSLContext.getInstance("SSL");
        sslContext.init(null, trustAllCerts, new java.security.SecureRandom());
        return sslContext;
    }
}

This issue happened under both h2 and h2c although the example code is h2. Btw, I didn't observe it happened for 10k or less, but it happened frequently for 100k. More information is in the bug comment.